It may come as a surprise that the freshwater mussel genus Actinonaias is actually restricted to Central America. After all, there are two species, “A.” ligamentina and “A.” pectorosa, that are well known in the eastern part of the United States. What the mucket is up with that? The short answer is that we now classify those two North American species in the genus Ortmanniana, and you can look at the taxonomy in our database (and elsewhere on this web site) or on the ITIS web site. Some longer answers follow.

How did we get a classification that lumped seven Central American species and two Mississippi Basin species into the genus Actinonaias?

Back at the dawn of the modern era of mussel classification, Simpson (1900) classified A. sapotalensis, A. medellina, and A. computata (Mesoamerican) in the genus Nephronaias, and he listed “A.” ligamentina (+ carinata) and “A.” pectorosa (+ perdix) (Nearctic) in the genus Lampsilis. When Ortmann (1912) examined the anatomy of A. sapotalensis, “A.” ligamentina, and “A.” pectorosa, he decided that the latter two species were morphologically distinct from the genus Lampsilis, but they were similar to A. sapotalensis. Indeed, as far as Ortmann was concerned about the soft parts, all three of these species were pretty similar to Obovaria. In order to update Simpson (1900), Ortmann (1912) moved “A”. ligamentina and “A.” pectorosa out of Lampsilis and into Nephronaias with A. sapotalensis.

Soon afterward and with more data, Frierson (1917) pointed out that other species of Nephronaias (such as the type species, N. plicatula) were anatomically more similar to genera like Elliptio and Pleurobema. According to Frierson, Nephronaias was thus composed of Elliptio-like species (elongate non-dimorphic shells with dark periostracum, ectobranchus) and Lampsilis-like species (sexually dimorphic shells with green rays, marsupium restricted to part of outer demibranchs). To correct this obviously confusing situation, Frierson (1917) applied Actinonaias to the lampsiline species like A. sapotalensis, A. medellina, and A. computata. It was Ortmann & Walker (1922) that decided that if A. sapotalensis was to be moved from Nephronaias to Actinonaias, so should “A.” ligamentina and “A.” pectorosa.

Besides the fact that there are several species of Actinonaias of questionable validity, no phylogenetic analysis has ever tested the monophyly (or non-monophyly) of the species formerly classified as Actinonaias. However, the biogeography of the two groups of species (Actinonaias sapotalensis, etc. vs. “A.” ligamentina and “A.” pectorsa) represents an unprecedented disjunction between the Mesoamerican and Nearctic faunal provinces. Separating the latter two species into Ortmanniana and leaving Actinonaias to A. sapotalensis and other Central American species makes biogeographical sense. The question that needs a phylogenetic answer is: Does A. sapotalensis share a more recent common ancestor with Ortmanniana ligamentina or other Mexican species? We hypothesize that our July 2015 Mussel of the Month belongs to a clade of Central American lampsilines.

Why did a classification that lumped seven Central American species and two Mississippi Basin species into the genus Actinonaias last as long as it did?

Very few researchers were ever confused by this classification because after Frierson (1917), very few researchers ever worried about Central American freshwater mussels. That is, for all intents and purposes, Actinonaias consisted of only two species of interest: “A.” ligamentina and “A.” pectorosa.

We should say “for MOST intents and purposes” because Frierson (1927) and Haas (1969) recognized that Ortmanniana in the Mississippi Basin was distinct from Actinonaias in Mexico. And, Watters et al. (2009) indicated the same thing. We imagine it is easy for systematists to get on board with the idea of splitting up the old Actinonaias. The sticking point is likely the required name change: Actinonaias stays with its type species, A. sapotalensis. The persistence of the use of Actinonaias for the species now in Ortmanniana has been a case of letting sleeping dogs lie.

We previously left these sleeping dogs to lie there in their likely paraphyly (Graf & Cummings, 2007), but no more. We have updated our classification to reflect the removal of Ortmanniana from Actinonaias. The MUSSEL Project Database has recently been incorporated into the Integrated Taxonomic Information System (ITIS), and the change is reflected there. Here’s to progress!